During startup of a voltage regulator, the reference voltage applied to the voltage regulator goes from 0V to 1.2V. If the reference voltage is turned on quickly, surge currents that are induced in the inductor associated with the voltage regulator can damage components of the voltage regulator and additionally cause a collapse of the input power supply by drawing too much current during the turn on process. It is greatly desired to reduce the surge currents during startup of the voltage regulator to prevent these actions from occurring. One method used to prevent surge currents has been the use of a soft start function that slowly ramps the reference voltage of the regulator from 0V to VREF. One problem with the slow startup method is the creation of a voltage offset with respect to VOUT on the FB pin due to initial movement of voltages in the COMP pin at startup enable (open loop operation) which is capacitively coupled to the FB pin. When an error amplifier within a voltage regulator is enabled, the voltage on the COMP pin may jump from 0V to 0.6V. Since the COMP pin is capacitively coupled to the FB pin, this will normally produce a voltage offset at the FB pin. The jump of voltage on the COMP pin is one source of the offset voltage at the FB pin.
During the actual slow start ramp process, the COMP pin will move from 0.6V up to a predetermined voltage level wherein operation of the voltage regulator enters linear operation (closed loop operation). While the COMP pin is moving up to the predetermined level, it will capacitively couple this voltage onto the FB pin. This is a second source of voltage injection into the FB pin. The voltage offset on the FB pin eventually discharges when the system enters closed loop operation. The discharge of the FB pin to VOUT reinforces with the soft start ramp resulting in an increased input current which can be sufficiently large to exceed the surge current specification for the regulator. The voltage charge is impressed onto the compensation loop which impresses the voltage upon the FB pin. When a pre-biased startup voltage is used, this can increase the amount of voltage impressed onto the compensation loop and the FB pin and further increase the surge currents on the FB pin during startup.